This invention relates to optical communications and specifically to a coupler with a periodic transmission response for coupling multiple radiation sources, and an optical amplifier using the coupler with a periodic transmission response.
Wave division multiplexing (WDM) increases bandwidth in optical communications by providing for communication over several wavelengths or channels. For long haul optical communications the optical signal must be periodically amplified. Current amplification schemes include Erbium doped fiber amplifiers (EDFA) and Raman amplifiers.
To maximize WDM capacity, it is desirable that the optical bandwidth of the system be as wide as possible. Thus, a wide range of optical signal wavelengths must be amplified. At the same time, it is desirable that the different optical signal wavelengths be amplified by about the same amount. Thus, it is desirable that the amplification gain profile of the amplification system should be both broad and relatively flat.
Raman amplification can provide a broad and relatively flat gain profile over the wavelength range used in WDM optical communications by using a plurality of different pump laser wavelengths. (See Y. Emori, “100 nm bandwidth flat-gain Raman Amplifiers pumped and gain-equalized by 12-wavelength channel WDM Diode Unit,” Electronic Lett., Vol. 35, no. 16, p. 1355 (1999). and F. Koch et. al., “Broadband gain flattened Raman Amplifiers to extend to the third telecommunication window,” OFC'2000, Paper FF3, (2000)). Raman amplifiers may be either distributed or discrete (See High Sensitivity 1.3 μm Optically Pre-Amplified Receiver Using Raman Amplification,” Electronic Letters, vol. 32, no. 23, p. 2164 (1996)). The Raman gain material in distributed Raman amplifiers is the transmission optical fiber, while a special spooled gain fiber is typically used in discrete Raman amplifiers.
FIG. 1 is a schematic of a portion of a typical optical communications system with a multiplexer 10 and an optical amplifier 12, such as a Raman amplifier. The multiplexer 10 receives a number of optical signals Sa, Sb, . . . Sz, respectively transmitted at optical communications wavelengths λa, λb, . . . λz. The multiplexer 10 multiplexes the optical signals and transmits the multiplexed signal along transmission optical fiber 14 to the optical amplifier 12. The optical amplifier 12 amplifies and transmits the optical signals.
FIG. 2 is a more detailed schematic of a typical distributed Raman optical amplifier 50 which can be employed as amplifier 12 in the multiplexer-amplifier system of FIG. 1. The amplifier 50 includes optical pump assembly 51 (shown enclosed by dashed lines) and transmission fiber 64. In this amplification scheme, the pump assembly 51 includes pump radiation sources 56 that collectively provide, for example, twelve different pump wavelengths λ1 through λ12. The pump radiation sources 56 are typically lasers that each emit radiation at a different wavelength of the wavelengths λ1 through λ12, respectively. Those skilled in the art will appreciate that more or fewer than 12 different pump wavelengths may be used in a given implementation. The wavelengths for pump radiation sources 56 (as well their pump powers) are selected based upon a number of different system design considerations, e.g., to provide an output signal-to-noise ratio (SNR) with minimal excursion. The radiation from the pump radiation sources 56 is then coupled or combined at a pump radiation combiner or coupler 54, e.g., a multiplexer, and the coupled radiation is output at pump radiation coupler output 58.
The coupled radiation has a coupled radiation profile that is a combination of the individual radiation profiles of the radiation input into the pump radiation combiner or coupler 54. The pump radiation profile that will be coupled with the optical signal to be amplified is therefore the coupled radiation profile in this case. The pump radiation profile is output from output 58 and then coupled at pump-signal combiner 60 with the optical signal 62. Optical signal 62, i.e., the data signal, propagates in the transmission optical fiber 64 in a direction opposite to the radiation, i.e., a counterpropagation direction, of the pump radiation profile. Alternatively, co-propagating or bi-directional pumping may also be employed. The optical signal is amplified along transmission optical fiber 62.
In general the criteria upon which the pump radiation wavelengths are selected will result in the identification of a specific set of pump wavelengths which are non-uniformly spaced apart across a particular bandwidth. Thus a spacing Δλ1 between a first pump wavelength and a second pump wavelength will be different than a spacing Δλ2 between the second pump wavelength and a third pump wavelength. The combiner 54, on the other hand, is typically designed to combine wavelengths that have a uniform, minimum spacing relative to one another.
Accordingly, it would be desirable to provide structures and techniques for selecting pump wavelengths in Raman amplified optical communication systems which permit the usage of off-the-shelf combiners or couplers in Raman amplification pump units.